Shell molding machines basically consist of a molding chamber, fed by a sand hopper established above it, which chamber is closed by a movable swiveling front plate and by a rear plate established on the end of an extractor piston, with the sand being compacted by the opposing pressure of the two plates.
In this way shells are cyclically obtained that form two half-molds and which are extracted with the aid of the extraction piston from the molding chamber and set against each other and aligned to form a row that will be carried, step by step, along the corresponding work stations, passing through a station where the molds are filled with the melt and proceeding while the part is cooled, to eventually reach the subsequent sub-processes.
Obviously, the successive shells forming the row define molds into which the melt is cast and thus must be perfectly aligned and maintained abutting and closed with a certain pressure that allows withstanding the melt pressure.
Traditionally, the movement of the row of shells along the installation is provided by the action of the extraction piston on the last shell manufactured, such that the extraction piston not only positions the shell against the end of the shell row but pushes on it to make the entire row advance. In each cycle, that is, each time a new shell is produced, the extraction piston makes the entire row advance one step, so that in successive cycles the row of shells advances step by step along the entire installation.
This advance method for the shells has two obvious drawbacks: on one hand, the wear suffered by the bottom area of the shells as they rub against the floor when traveling through the installation; and, on the other, the compression of the shells as they are pushed by the extraction piston to make the entire row advance.
Also known are installations wherein the displacement and advance of the shells along the installation is initially effected by the piston and later by a conveyor system. An installation of this type is for example described in European Patent n° 0 693 337, which describes a conveyor system consisting of a series of longitudinal rails that can move forward or backward, towards or away from the molding station. In this way the extraction piston pushes the last shell obtained until it is placed on the rails of the conveyor system, leaving it against the row of previously formed shells, so that the conveyor system provides the stepwise advance of the row of shells thereafter.
Thus, as the extraction piston performs the first displacement of the shell to place it on the conveyor, the operation of the extraction piston depends on the position and working cycle of the conveyor system, so that it cannot push the shell out of the molding station until the conveyor system is in the position for receiving the shells. In this way the cycle or operation of the molding station is conditioned by the motion of the row of shells, so that any interruption or stoppage of the subsequent stations will affect the output of the molding station.
Specifically, the conveyor system consists of longitudinal rails free to travel over rollers, making the shell row advance one step, after which lateral clamps independent of the longitudinal rails firmly secure the last shell as the longitudinal rails of the conveyor recede until reaching the end of the molding station, so that the extraction piston can place another shell on the conveyor. With this type of installation the shells are subject to friction against the rails that form the conveyor system, so that in this case there would also be wear damaging the shell that may affect the quality of the parts obtained.
Furthermore, the lateral clamps must apply a great pressure on the last shell, as they must prevent the recession of the entire row of shells when the conveyor rails recede, thus resulting in compressions and deformations of this last shell that also reduce the quality of the parts obtained.
In addition, the following or pushing by the extraction piston in the initial movement of the shell until it is placed on the conveyor system requires a perfect synchronization of the motions of the conveyor system and the extraction piston, as otherwise slight differences in the position and/or speed may occur that would lead to friction between the shells, which would in turn result in a wear of the sand mold and eventually cause defects in the part.
French Patent FR 2 160 747 refers to a conveyor device for a vertical shell molding machine wherein the last shell manufactured by the shell molding machine is expelled by an extraction piston against a row of shells on a shell conveyor system. The conveyor device consists of a combination of fixed and mobile beams, said mobile beams providing the vertical movement of the row of shells at the same time the last shell is being pressed against the row of shells. This invention shows the same problems as the European Patent previously mentioned.